Mechanisms for forming image-sensor device with deep-trench isolation structure

ABSTRACT

An image-sensor device is provided. The image-sensor device includes a substrate having a front side and a back side. The image-sensor device also includes a radiation-sensing region operable to detect incident radiation that enters the substrate through the back side. The image-sensor device further includes a deep-trench isolation structure extending from the back side towards the front side. The deep-trench isolation structure includes a dielectric layer, and the dielectric layer contains hafnium or aluminum.

CROSS REFERENCE

This Application is a Continuation application of U.S. patentapplication Ser. No. 15/606,950, filed on May 26, 2017, which is aContinuation application of U.S. patent application Ser. No. 15/093,285,filed on Apr. 7, 2016, which is a Continuation application of U.S.patent application Ser. No. 14/822,575, filed on Aug. 10, 2015, which isa Divisional of U.S. application Ser. No. 14/016,949, filed on Sep. 3,2013, the entire of which are incorporated by reference herein.

BACKGROUND

Semiconductor image sensors are used to sense radiation such as light.Complementary metal-oxide-semiconductor (CMOS) image sensors (CIS) andcharged-coupled device (CCD) sensors are widely used in variousapplications such as digital camera or mobile phone camera applications.These devices utilize an array of pixels in a substrate, includingphotodiodes and transistors, to absorb radiation projected toward thesubstrate and convert the sensed radiation into electrical signals.

A backside-illuminated (BSI) image-sensor device is one type ofimage-sensor device. The BSI image-sensor device is used for sensing avolume of light projected towards a backside surface of a substrate(which supports the image sensor circuitry of the BSI image-sensordevice). The pixel grid is located at a front side of the substrate, andthe substrate is thin enough so that light projected towards thebackside of the substrate can reach the pixel grid. The BSI image-sensordevice provides a high fill factor and reduced destructive interference,as compared to frontside-illuminated (FSI) image-sensor devices.Although existing BSI image-sensor devices and methods of fabricatingthese BSI image-sensor devices have been generally adequate for theirintended purposes, as device scaling down continues, they have not beenentirely satisfactory in all respects.

BRIEF DESCRIPTION OF THE DRAWING

For a more complete understanding of the present disclosure, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings.

FIG. 1 shows a flow chart illustrating a method for fabricating animage-sensor device, in accordance with some embodiments.

FIGS. 2-9 are diagrammatic fragmentary cross-sectional views of animage-sensor device at various stages of fabrication, in accordance withsome embodiments.

DETAILED DESCRIPTION

It is to be understood that the following disclosure provides manydifferent embodiments, or examples, for implementing different featuresof the disclosure. Specific examples of components and arrangements aredescribed below to simplify the present disclosure. These are, ofcourse, merely examples and are not intended to be limiting. Moreover,the performance of a first process before a second process in thedescription that follows may include embodiments in which the secondprocess is performed immediately after the first process, and may alsoinclude embodiments in which additional processes may be performedbetween the first and second processes. Various features may bearbitrarily drawn in different scales for the sake of simplicity andclarity. Furthermore, the formation of a first feature over or on asecond feature in the description that follows may include embodimentsin which the first and second features are formed in direct contact, andmay also include embodiments in which additional features may be formedbetween the first and second features, such that the first and secondfeatures may not be in direct contact. In addition, the like elements invarious figures and embodiments are identified by the same or similarreference numerals.

The image-sensor device according to the present disclosure is abackside-illuminated (BSI) image-sensor device. The BSI image-sensordevice includes a charge-coupled device (CCD), a complementary metaloxide semiconductor (CMOS) image sensor (CIS), an active-pixel sensor(APS) or a passive-pixel sensor. The image-sensor device may includeadditional circuitry and input/outputs that are provided adjacent to thegrid of pixels for providing an operation environment of the pixels andfor supporting external communication with the pixels.

Illustrated in FIG. 1 is a flowchart of a method for fabricating animage-sensor device according to some embodiments. Referring to FIG. 1,the method 10 begins with a block 11 in which a substrate having a frontsurface and a back surface is provided. The substrate includes aradiation-sensing region and a doped isolation region adjacent to theradiation-sensing region. The method 10 continues with a block 13 inwhich a deep-trench isolation structure is formed. The deep-trenchisolation structure includes a trench extending from the back surfaceand a negatively charged film covering the trench. The method 10continues with a block 15 in which a reflective piece is formed over thedeep-trench isolation structure.

FIGS. 2-9 are diagrammatic fragmentary cross-sectional views of animage-sensor device at various stages of fabrication, in accordance withsome embodiments. It is understood that FIGS. 2-9 have been simplifiedfor a better understanding of embodiments of the present disclosure.

Referring to FIG. 2, the image-sensor device 100 includes a substrate102. The substrate 102 is a device substrate. The substrate 102 may be asemiconductor substrate. The substrate 102 may be a silicon substratedoped with a P-type dopant such as boron, in which case the substrate102 is a P-type substrate. Alternatively, the substrate 102 could beanother suitable semiconductor material. For example, the substrate 102may be a silicon substrate doped with an N-type dopant such asphosphorous or arsenic, in which case the substrate is an N-typesubstrate. The substrate 102 may include other elementary semiconductormaterials such as germanium or diamond. The substrate 102 may optionallyinclude a compound substrate and/or an alloy semiconductor. Further, thesubstrate 102 may include an epitaxial layer (epi layer), may bestrained for performance enhancement, and may include asilicon-on-insulator (SOI) structure.

The substrate 102 has a front surface 104 (also referred to as afrontside) and a back surface 106 (also referred to as a backside). Fora BSI image sensor device such as the image-sensor device 100, incidentradiation enters the substrate 102 through the back surface 106. In someembodiments, the substrate 102 has a thickness ranging from about 500 μmto about 1000 μm. The substrate 102 is fabricated with front endprocesses, in accordance with some embodiments. For example, thesubstrate 102 includes various regions, which may include a pixelregion, a periphery region, a bonding pad region, and a scribe lineregion. For the sake of simplicity, only a portion of the pixel regionis shown in FIGS. 2 to 9.

The pixel region includes radiation-sensing regions 108 and dopedisolation regions 110. The radiation-sensing regions 108 are doped witha doping polarity opposite from that of the substrate 102. Theradiation-sensing regions 108 are formed by one or more implantationprocesses or diffusion processes. The radiation-sensing regions 108 areformed adjacent to or near the front surface 104 of the substrate 102.Although only a portion of the pixel region is shown in FIG. 2, thepixel region may further include pinned layer photodiodes, photodiodegates, reset transistors, source follower transistors, and transfertransistors. For the sake of simplicity, detailed structures of theabove features are not shown in figures of the present disclosure.

The radiation-sensing regions 108 are operable to sense incidentradiation that enters the pixel region from the back surface 106. Theincident radiation may be visual light. Alternatively, the incidentradiation may be infrared (IR), ultraviolet (UV), X-ray, microwave,other suitable types of radiation, or a combination thereof.

The doped isolation regions 110 are adjacent to the radiation-sensingregions 108, in accordance with some embodiments. The doped isolationregions 110 are formed adjacent to or near the front surface 104. Eachpair of neighboring radiation-sensing regions 108 is separated from oneanother by one of the respective doped isolation regions 110. The dopedisolation regions 110 are doped with a doping polarity the same as thatof the substrate 102. In some embodiments, the doping concentration ofthe doped isolation regions 110 is higher than that of the substrate102. For example, the doping concentration of the doped isolationregions 110 may be in a range of about 1E16 per cm³ to about 1E20 percm³. The doped isolation regions 110 are formed by one or moreimplantation processes or diffusion processes.

As shown in FIG. 2, isolation features 112 are formed in the dopedisolation regions 110, in accordance with some embodiments. Theisolation features 112 are formed adjacent to or near the front surface104 of the substrate 102. In some embodiments, the isolation features112 are used to define predetermined regions of the radiation-sensingregions 108 and doped isolation regions 110. Therefore, the isolationfeatures 112 may be formed before forming the radiation-sensing regions108 and doped isolation regions 110. In some embodiments, the dopedisolation regions 110 are aligned with the isolation features 112.

The isolation features 112 include shallow trench isolation (STI)structures and/or local oxidation of silicon (LOCOS) structures. In someembodiments, some active or passive features, such as MOSFET or junctioncapacitor, are formed in the doped isolation regions 110, according todesign needs and manufacturing concerns. The active or passive featuresin the doped isolation regions 110 are surrounded and protected by theisolation features 112. The thickness of the isolation features 112 isgreater than that of the active or passive features in the dopedisolation regions 110. In some embodiments, the thickness of theisolation features 112 is in a range from about 100 angstrom to about5000 angstrom.

In some embodiments, the isolation features 112 are formed by formingtrenches in the substrate 102 from the front surface 104 and filling adielectric material into the trenches. The dielectric material mayinclude silicon oxide, silicon nitride, silicon oxynitride, a low-kmaterial, or another suitable dielectric material. A chemical mechanicalpolishing (CMP) process may be performed to planarize the surface of thedielectric material filling the trenches.

As shown in FIG. 2, the image-sensor device 100 may further include aninterconnection structure 114 formed over the front surface 104 of thesubstrate 102. The interconnection structure 114 includes a number ofpatterned dielectric layers and conductive layers that couple to variousdoped features, circuitry, and input/output of the image-sensor device100. The interconnection structure 114 includes an interlayer dielectric(ILD) and a multilayer interconnection (MLI) structure. The MLIstructure includes contacts, vias and metal lines. For the purpose ofillustration, a number of conductive lines 116 and vias/contacts 118 areshown in FIG. 2, it being understood that the conductive lines 116 andvias/contacts 118 are merely exemplary. The actual positioning andconfiguration of the conductive lines 116 and vias/contacts 118 may varydepending on design needs and manufacturing concerns.

Referring to FIG. 3, a buffer layer 120 is formed on the interconnectionstructure 114, in accordance with some embodiments. The buffer layer 120may include a dielectric material such as silicon oxide. Alternatively,the buffer layer 120 may include silicon nitride. The buffer layer 120may be deposited by chemical vapor deposition (CVD), physical vapordeposition (PVD), or other suitable techniques. The buffer layer 120 maybe planarized to form a smooth surface by a CMP process.

Afterwards, a carrier substrate 122 is bonded with the substrate 102through the buffer layer 120. Therefore, the processing of the backsurface 106 of the substrate 102 can be performed. In some embodiments,the carrier substrate 122 is similar to the substrate 102 and includes asilicon material. Alternatively, the carrier substrate 122 may include aglass substrate or another suitable material. The carrier substrate 122may be bonded to the substrate 102 by molecular forces (direct bonding),optical fusion bonding, metal diffusion bonding, anodic bonding, or byother suitable bonding techniques. The buffer layer 120 provideselectrical isolation between the substrate 102 and carrier substrate122. The carrier substrate 122 provides protection for the variousfeatures formed on the front surface 104 of the substrate 102. Thecarrier substrate 122 also provides mechanical strength and support forprocessing the back surface 106 of the substrate 102 as discussed below.

After the carrier substrate 122 is bonded, a thinning process is thenperformed to thin the substrate 102 from the back surface 106. Thethinning process may include a mechanical grinding process. Afterwards,an etching chemical may be applied over the back surface 106 ofsubstrate 102 to further thin the substrate 102 to a thickness which ison the order of a few microns. In some embodiments, the thickness of thesubstrate 102, after being thinned, is in a range from about 1 μm toabout 100 μm.

Common image-sensor device defects include optical cross-talk,electrical cross-talk and dark current. The defects become more seriousas the image pixel sizes and the spacing between neighboring imagepixels continues to shrink. Optical cross-talk refers to photoninterference from neighboring pixels that degrades the light-sensingreliability and accuracy of the pixels. Dark current may be defined asthe existence of pixel current when no actual illumination is present.In other words, the dark current is the current that flows through thephotodiode when no photons are entering the photodiode. White pixelsoccur where an excessive amount of current leakage causes an abnormallyhigh signal from the pixels. In the image-sensor device 100 shown inFIG. 3, the doped isolation regions 110 have a doping polarity oppositeto that of the radiation-sensing regions 108 to reduce the dark currentand white pixel defects. However, the doped isolation regions 110 alonemay not effective enough to prevent dark current and white pixeldefects. In addition, the doped isolation regions 110 could not resolvethe optical cross-talk defect due to the similar refractive index of theradiation-sensing regions 108 and doped isolation regions 110.

Referring to FIG. 4, an etching process is performed on the back surface106 of the substrate 102 to form a number of openings 124 (ortrenches/recesses). The etching process includes a dry etching process.An etching mask (for example a hard mask, not illustrated herein) may beformed before the etching process is performed. Each of the openings 124has a width W₁ at the back surface 106 of the substrate 102. The widthW₁ may be smaller than or substantially equal to that of the dopedisolation regions 110. The openings 124 may have a rectangular shape, atrapezoidal shape, or another suitable shape. In some embodiments, eachof the openings 124 extends over half of the thickness of the substrate102 but does not reach the isolation features 112. Accordingly, activeor passive features surrounded by the isolation features 112 may be notdamaged by the etching process. In some embodiments, the depth of theopenings 124, measured from the back surface 106 of the substrate 102,is in a range from about 1 μm to about 10 μm. The depth of the openings124 may be adjusted by time control without using an etching stop layer.These openings 124 are used for forming deep-trench isolation (DTI)structures, which will be discussed in more detail below.

Referring to FIG. 5, a negatively charged film 126 is formed over theback surface 106 of substrate 102, in accordance with some embodiments.The negatively charged film 126 may conformally cover the back surface106, including covering interior surfaces of the openings 124 in aconformal manner. The negatively charged film 126 has a greater overallnegative charge than traditional dielectric films. The negative chargeincreases hole accumulation at an interface of the negatively chargedfilm 126 and creates a depletion region at or close to the interface ofthe negatively charged film 126 and doped isolation regions (i.e.,p-type) 110 of the substrate 102 around the radiation-sensing regions108. The depletion region reduces dark current and/or white pixels.

According to one or more embodiments, the negatively charged film 126 isa high-k metal oxide. The high-k metal oxide may be a hafnium oxide,aluminum oxide, zirconium oxide, magnesium oxide, calcium oxide, yttriumoxide, tantalum oxide, strontium oxide, titanium oxide, lanthanum oxide,barium oxide or other metal oxides that can form a high-k film usingexisting semiconductor deposition technologies. The high-k metal oxidemay be deposited using a CVD process or a PVD process. The CVD processmay be plasma enhanced chemical vapor deposition (PECVD) includingICPECVD, a low pressure chemical vapor deposition (LPCVD), or an atomiclayer deposition (ALD) with or without plasma. These processes may betuned to favor an accumulation of negative charge by varying the processparameters including various flow rates and power parameters, and mayinvolve a treatment step after the film deposition to increase negativecharge. The resulting high-k metal oxide film may have an oxygen-richcomposition with negatively charged interstitial oxygen atoms and/ordangling/broke metal oxide bonds, both of which results in a cumulatednegative charge. The cumulated negative charge may be around 5E9 toaround 1E14 per cm², or greater than about 1E10/cm². In other words, thetotal charge (Qtot) for the layer is around −5E9 to around −1E14 percm², or more negative than about 1E10/cm².

According to other embodiments, the negatively charged film 126 is asilicon nitride or nitride dielectric. The nitride material may be anitrogen-rich silicon nitride or another nitrogen-rich dielectric film,such as tantalum nitride, titanium nitride, hafnium nitride, aluminumnitride, magnesium nitride, or other metal nitrides that can be formedusing existing semiconductor deposition technologies. The nitridematerial may be deposited using a CVD technique or a PVD technique. TheCVD process may be a PECVD including ICPECVD, an LPCVD, or an ALD withor without plasma. In some embodiments, the negatively charged film is aplasma nitrided material. The plasma nitridation may occur during orafter film deposition in an after treatment, if a nonplasma depositiontechnique is used, by using a plasma containing nitrogen ions. Theplasma nitridation creates a nitrogen-rich film with a cumulatednegative charge. In some embodiments, the negative charge is increasedby a thermal or plasma treatment with ammonia. The cumulated negativecharge is around 1E9 to around 1E13 per cm², or greater than about5E9/cm². In other words, the total charge (Qtot) for the layer is around−1E9 to around −1E13 per cm², or more negative than about 5E9/cm². Insome embodiments, the negatively charged film 126 has a thicknessranging from about 1 nm to about 500 nm. In some other embodiments, thenegatively charged film 126 has a thickness ranging from about 1 nm toabout 100 nm.

Afterwards, referring to FIG. 6, a dielectric material 128 is depositedover the back surface 106 of the substrate 102, in accordance with someembodiments. The dielectric material 128 fills the remaining spaces ofthe openings 124. In some embodiments, the dielectric material 128includes silicon oxide, silicon nitride, silicon oxynitride, spin onglass (SOG), low-k dielectric, or another suitable dielectric material.The dielectric material 128 may be deposited by CVD, PVD, or anothersuitable depositing technique. In some embodiments, a portion of thedielectric material 128 outside the openings 124 is thinned andplanarized. In the following discussion, the openings 124 and portionsof the negatively charged film 126 and dielectric material 128 in theopenings 124 are collectively referred to as deep-trench isolationstructures 130.

Afterwards, referring to FIG. 7, a reflective grid 132 is formed overthe substrate 102, in accordance with some embodiments. For example, thereflective grid 132 is formed on the dielectric material 128. Each pieceof the reflective grid 132 is aligned with one of the respectivedeep-trench isolation structures 130. In some embodiments, thereflective grid 132 is formed of a metal material, such as aluminum,tungsten, copper, tantalum, titanium, alloys thereof, or combinationsthereof. Each piece of the reflective grid 132 may have a rectangularshape, a reverse trapezoidal shape, reverse triangle shape, or anothersuitable shape. In some embodiments, each piece of the reflective grid132 have a thickness T ranging from about 100 Å to about 15000 Å. Thereflective grid 132 is formed by a suitable deposition process and thenpatterned. The deposition process includes electroplating, sputtering,CVD, PVD or other suitable depositing techniques. The CVD process may bea PECVD including ICPECVD, an LPCVD, or an ALD with or without plasma.

In some embodiments, each piece of the reflective grid 132 has a widthW₂ at the back surface 106. For example, the width W₂ is in a range from10 to 1000 nm. The width W₂ is substantially equal to or greater thanthe width W₁ of the deep-trench isolation structures 130 to cover thedeep-trench isolation structures 130. Therefore, the reflective grid 132prevents the nearly vertical incident radiation from travelling into thedeep-trench isolation structures 130. The nearly vertical incidentradiation that travels into the deep-trench isolation structures 130 maybe refracted to adjacent radiation-sensing regions 108, and undesiredphoto cross-talk would occur.

Afterwards, referring to FIG. 8, a transparent filling layer 134 isdeposited over the back surface 106 of the substrate 102, in accordancewith some embodiments. The transparent filling layer 134 may be made ofsilicon oxide, silicon nitride, or suitable polymers, and may be formedby suitable techniques, such as CVD, PVD, or combinations thereof. Insome embodiments, the transparent filling layer 134 has a thicknessgreater than that of the reflective grid 132. Accordingly, thetransparent filling layer 134 covers the reflective grid 132 andprovides a smooth surface. For example, the transparent filling layer134 has a thickness ranging from about 10 angstrom to about 1000angstrom. In some embodiments, the transparent filling layer 134functions as an antireflective layer of the image-sensor device 100. Theantireflective layer serves to reduce reflection of the incidentradiation projected toward the back surface 106 of the image-sensordevice 100.

Thereafter, referring to FIG. 9, a color filter layer 136 is formed overthe transparent filling layer 134, in accordance with some embodiments.The color filter layer 136 supports the filtering of incident radiationhaving a particular range of wavelengths, which may correspond to aparticular color of light, for example, red, green, or blue. The colorfilter layer 136 may be used to allow only light having a predeterminedcolor to reach of the radiation-sensing regions 108. Afterwards, a microlens layer 138 may be formed over the color filter layer 136 fordirecting incident radiation toward the radiation-sensing regions. Themicro lens layer 138 may be positioned in various arrangements and havevarious shapes depending on the refractive index of the material usedfor the micro lens layer 138 and/or the distance between the micro lenslayer 138 and the radiation-sensing regions 108. Alternatively, theposition of the color filter layer 136 and micro lens layer 138 may bereversed such that the micro lens layer 138 may be disposed between theback surface 106 of the substrate 102 and color filter layer 138.

Embodiments of mechanisms for forming an image-sensor device aredescribed. Deep-trench isolation structures, which are formed from theback surface of the substrate and include a negatively charged film, mayfurther reduce the dark current and white pixel defects. In addition, areflective grid formed over the deep-trench isolation structures mayprevent incident radiation from traveling into the deep-trench isolationstructures. Therefore, the photo cross-talk defect is also reduced orprevented.

In accordance with some embodiments, an image-sensor device is provided.The image-sensor device includes a substrate having a front surface anda back surface. The image-sensor device also includes aradiation-sensing region operable to detect incident radiation thatenters the substrate through the back surface. The image-sensor devicefurther includes a doped isolation region formed in the substrate andadjacent to the radiation-sensing region. In addition, the image-sensordevice includes a deep-trench isolation structure formed in the dopedisolation region. The deep-trench isolation structure includes a trenchextending from the back surface into the doped isolation region and anegatively charged film covering an interior surface of the trench.

In accordance with some embodiments, an image-sensor device is provided.The image-sensor device includes a substrate having a front surface anda back surface. The image-sensor device also includes a plurality ofradiation-sensing regions formed in the substrate. The image-sensordevice further includes a plurality of doped isolation regions formed inthe substrate. Each pair of neighboring radiation-sensing regions isseparated from one another by one of the respective doped isolationregions. In addition, the image-sensor device includes a plurality oftrenches extending from the back surface into the doped isolationregions. The image-sensor device also includes a negatively charged filmcovering interior surfaces of the trenches and a dielectric materialover the negatively charged film and filling the trenches. Theimage-sensor device further includes a reflective grid formed over theback surface of the substrate, and each piece of the reflective grid isaligned with one of the respective trenches.

In accordance with some embodiments, a method of fabricating animage-sensor device is provided. The method includes providing asubstrate having a front surface and a back surface. The method alsoincludes forming a radiation-sensing region and a doped isolation regionadjacent to the front surface, and the doped isolation region isadjacent to the radiation doped region. The method further includesforming a trench in the doped isolation region from the back surface. Inaddition, the method includes forming a negatively charged film over theback surface and covering an interior surface of the trench.

Although embodiments of the present disclosure and their advantages havebeen described in detail, it should be understood that various changes,substitutions and alterations can be made herein without departing fromthe spirit and scope of the disclosure as defined by the appendedclaims. For example, it will be readily understood by those skilled inthe art that many of the features, functions, processes, and materialsdescribed herein may be varied while remaining within the scope of thepresent disclosure. Moreover, the scope of the present application isnot intended to be limited to the particular embodiments of the process,machine, manufacture, composition of matter, means, methods andoperations described in the specification. As one of ordinary skill inthe art will readily appreciate from the disclosure of the presentdisclosure, processes, machines, manufacture, compositions of matter,means, methods, or operations, presently existing or later to bedeveloped, that perform substantially the same function or achievesubstantially the same result as the corresponding embodiments describedherein may be utilized according to the present disclosure. Accordingly,the appended claims are intended to include within their scope suchprocesses, machines, manufacture, compositions of matter, means,methods, or operations.

What is claimed is:
 1. An image-sensor device, comprising: a substratehaving a front side and a back side; a radiation-sensing region operableto detect incident radiation that enters the substrate through the backside; and a deep-trench isolation structure extending from the back sidetowards the front side, wherein the deep-trench isolation structurecomprises a dielectric layer, and the dielectric layer containsaluminum.
 2. The image-sensor device as claimed in claim 1, wherein thedielectric layer comprises an aluminum oxide film, or an aluminumnitride film.
 3. The image-sensor device as claimed in claim 1, whereinthe dielectric layer further extends over the radiation-sensing region.4. The image-sensor device as claimed in claim 1, wherein the dielectriclayer is in direct contact with the radiation-sensing region.
 5. Theimage-sensor device as claimed in claim 1, wherein the dielectric layerhas a thickness in a range from about 1 nm to about 100 nm.
 6. Theimage-sensor device as claimed in claim 1, further comprising anisolation feature extending from the front side towards the back side ofthe substrate.
 7. The image-sensor device as claimed in claim 6, whereinthe deep-trench isolation structure extends towards the isolationfeature.
 8. The image-sensor device as claimed in claim 6, furthercomprising a dielectric film over the dielectric layer and extendingtowards the isolation feature, wherein the dielectric layer is betweenthe dielectric film and the substrate.
 9. The image-sensor device asclaimed in claim 8, wherein the dielectric layer and the dielectric filmare made of different materials.
 10. The image-sensor device as claimedin claim 9, further comprising a reflective piece over the dielectricmaterial, wherein the reflective piece is substantially aligned with theisolation feature.
 11. An image-sensor device, comprising: a substratehaving a front side and a back side; a radiation-sensing region operableto detect incident radiation that enters the substrate through the backside; a doped isolation region formed in the substrate and adjacent tothe radiation-sensing region; and a deep-trench isolation structureextending from the back side towards the front side of the substrate,wherein the deep-trench isolation structure is in direct contact withthe doped isolation region, and a portion of the doped isolation regionis between a top end of the deep-trench isolation structure and thefront side of the substrate.
 12. The image-sensor device as claimed inclaim 11, wherein the deep-trench isolation structure comprises adielectric layer.
 13. The image-sensor device as claimed in claim 12,wherein the dielectric layer is in direct contact with an interfacebetween the radiation-sensing regions and the doped isolation region.14. The image-sensor device as claimed in claim 12, wherein thedielectric layer comprises a hafnium-containing oxide film, ahafnium-containing nitride film, an aluminum-containing oxide film, oran aluminum-containing nitride film.
 15. The image-sensor device asclaimed in claim 11, further comprising an isolation feature extendsfrom the front side of the substrate towards the deep-trench isolationstructure.
 16. An image-sensor device, comprising: a substrate having afront side and a back side; a radiation-sensing region operable todetect incident radiation that enters the substrate through the backside; a deep-trench isolation structure in the substrate, wherein thedeep-trench isolation structure is separated from the radiation-sensingregion by a portion of the substrate, the deep-trench isolationstructure comprises a dielectric layer and an inner portion, thedielectric layer and the inner portion are made of different materials,the dielectric layer extends along sidewalls of the inner portion, thedielectric layer is in direct contact with the portion of the substrate,and the dielectric layer contains hafnium or aluminum.
 17. Theimage-sensor device as claimed in claim 16, wherein the dielectric filmcomprises a hafnium oxide film, a hafnium nitride film, an aluminumoxide film, or an aluminum nitride film.
 18. The image-sensor device asclaimed in claim 16, further comprising a doped isolation region formedin the substrate and surrounding the radiation-sensing region, whereinthe doped isolation region is in direct contact with the deep-trenchisolation structure.
 19. The image-sensor device as claimed in claim 16,further comprising a shallow trench isolation structure extending fromthe front side of the substrate towards the deep-trench isolationstructure.
 20. The image-sensor device as claimed in claim 16, furthercomprising a doped isolation region formed in the substrate and adjacentto the radiation-sensing region, wherein a portion of the dopedisolation region is between a top end of the deep-trench isolationstructure and the front side of the substrate.